The in vivo mouse embryoid body/teratocarcinoma system is being utilized as a model to study the molecular mechanisms of cellular determination and differentiation. In vitro translation systems (wheat germ S-30 and rabbit reticulocyte lysate) will permit the analysis of the patterns of protein synthesis during the differentiation process from embryoid bodies to teratomas. In vitro translation systems will also allow the characterization, of total poly(A)-containing RNA and cytoplasmic ribosomal-associated poly(A)-containing RNA preparations by analysis of synthesized products. Isolation of specific mRNA's (e.g. Alpha feto protein mRNA) will permit the analysis of gene transcription with cDNA probes during differentiation and the analysis of precursors of the final protein products.